A reference list is appended hereto. Citation numbers identified in parentheses refer to this reference list. All of the teachings of the references identified in the reference list are incorporated herein by reference.
A visual prosthesis is a device that captures aspects of the visual environment and uses this information to stimulate nerves within the visual pathway to influence vision. (40, 60, 61, 76, 77, 85, 114, 115, 116, 136, 142). A visual prosthesis may be placed within the eye or at some location on the path toward or within the visual part of the brain. Visual prosthetic devices within the eye can be positioned on the inner surface of the retina (i.e. epi-retinal) or under the retina (sub-retinal). Every option for placement provides certain advantages and creates certain disadvantages that must be addressed if vision is to be created in a manner that would be useful to a blind patient.
A retinal prosthesis is designed to replace the function of degenerated nerve cells of an eye that causes in blindness. The retinal prosthesis has the capability to stimulate surviving nerve cells in a manner designed to convey information about the visual world. The surviving nerve cells of the eye carry the artificially induced nerve signals to the visual part of the brain through the optic nerve. The broad concept of utilizing a retinal prosthesis to restore vision to the blind was first described in U.S. Pat. No. 2,760,483 to Tassicker and later in U.S. Pat. No. 4,628,933 to Michelson, the teachings of both of which are incorporated herein by reference.
At least two significant forms of blindness occur because of a loss of the photoreceptive cells of the retina, i.e. those cells that normally convert the energy from light that enters the eye into a nerve signal that is carried to the brain. Age-related macular degeneration is the leading cause of blindness in the industrialized world, and retinitis pigmentosa is the leading cause of inherited blindness throughout the world. Age-related macular degeneration results in a loss of central vision, which eliminates a person's ability to read or recognize faces. Retinitis pigmentosa results in a slow loss of peripheral and then central vision. Patients with retinitis pigmentosa are more severely affected because of the loss of both central and peripheral vision. A worthy goal of treatment for patients with retinitis pigmentosa is restoration of some peripheral vision, which would enhance a patient's ability to ambulate independently and more safely in unfamiliar environments. A retinal prosthesis has the potential to restore vision to patients with these and potentially other forms of blindness caused by a dysfunctional retina.
Retinal prostheses can be broadly divided into two categories: epi-retinal (30, 63, 64, 66, 81, 83, 105, 106, 109, 111, 136, 142) and sub-retinal. (14, 15, 26, 28, 29, 119, 120, 121, 129–132). Epi-retinal devices are placed on or near the inner surface of the retina, that is, the side which is first exposed to incoming light rays and along which the nerve fibers of the ganglion cells pass on their way to the optic nerve. Sub-retinal devices are placed under the retina, between the retina and the underlying retinal pigment epithelium or other deeper tissues. Although devices in either location are capable of effectively stimulating retinal nerve cells, there are advantages and potential disadvantages to each strategy. One very significant advantage of a sub-retinal prosthesis is the opportunity to implant the device by approaching the sub-retinal space from outside of the eye (i.e. ab externo, through the sclera covering the back of the eye), rather than having to perform any (or any significant) surgery within the center of the eye, which is much more likely to result in chronic inflammation, infection or a host of other problems that might compromise the safe implantation or effectiveness of a prosthesis.
Retinal prostheses can also be divided into the means by which they receive power for their operation. Any retinal prosthesis must embody the means to: 1) capture a visual image; 2) translate the details of a captured visual image into a pattern of stimulation of the retina; and 3) obtain sufficient power to both operate the electronics and stimulate the retina. Retinal prostheses have been disclosed which use electromagnetic energy obtained either from light (119–132) or radiofrequency (136–142) transmission.
A third division of retinal prostheses is by the location at which the image from the external world is acquired. An external camera located on a pair of glasses or elsewhere outside the body is one possibility, while an imaging system implanted within the eye is another. An external camera has the advantage of faithfully capturing details of a visual scene but this signal must be then be transformed to a pattern of stimulation commands sent to the implanted prosthesis. An additional disadvantage is that, in simple implementations, the view the patient sees is determined artificially by motion of the external camera rather than by the natural motion of the eye. An internal imaging system has the advantage of allowing the direction of the eye to determine what the patient sees, but the imaging system must be implanted within the body and the exact stimulation pattern applied to the retina is not known from outside.
The goal of creating “useful” vision is extremely challenging, primarily because of the intricacy of human vision, the complexity of the nerve circuitry that provides vision, and because of the potential for the body to reject implantation of a foreign object, especially near the retina, which is as delicate and complex as the rest of the brain. Creation of anything like normal vision by a prosthesis would require the ability to generate images that contain at least the attributes of spatial detail (i.e. to permit reading or other fine work), color vision and variations of contrast (which improve the resolution in an image). These qualities are normally provided by nerve cells in the center of the retina (i.e. the macula) and, as such, these attributes of vision can be considered to represent “central vision”. The area of the macula, however, is only a small fraction of the total area of the retina. The majority of primate vision is actually “peripheral” vision that is designed simply to detect the presence of objects in the environment. Once alerted, a person would then move the eye so that the center of the retina (with its much higher density of nerve cells and hence greater capacity to resolve visual detail) fixates on the target of interest. Peripheral vision provides an especially good ability to detect motion because of the presence of nerve cells with relatively large receptive fields (i.e. the area within which the cells will respond to moving objects). Ideally, a visual prosthesis should be designed in a manner to permit restoration of both central and peripheral vision, depending upon the needs of individual patients. A visual prosthesis could also be used to influence light-induced circadian rhythms, modulate pupillomotor responses to control the amount of light entering the eye, or, if a prostheses were placed in homologous regions of the retina in both eyes, provide stereopsis (i.e. depth perception).
The requirements differ for a prosthesis able to generate useful central vision as compared with useful peripheral vision. The relatively high resolution of central vision can only be delivered by a prosthesis if the electrode array that abuts the retina contains a relatively high density of stimulating elements. Driving the larger number of electrodes needed for this purpose imposes greater demands upon the power delivery of the prosthesis, which must in turn obtain that power from an artificial, external source, given that the intensity of the incident light that illuminates the retina is woefully insufficient to electrically activate nerve cells (which evolved to respond with high sensitivity to light, not electricity)). The relatively intense power source required has the potential to damage the retina because of the unavoidable loss of energy at a secondary coil of a radiofrequency system that produces heat. Similarly, attempts to obtain sufficient energy from light entering the eye (as from a prosthetic device that provides electronically filtered light from the outside) also have the potential to damage the retina because of the well-described phototoxicity that would occur with the levels of light needed to power a prosthesis.
Restoring peripheral vision (i.e. mainly detection of objects in the environment) is also a challenging goal, although the challenges differ in several respects from those described above with respect to central vision. The normal range of peripheral vision equals approximately 150 degrees in each eye (90 degrees from the center to the farthest extent to the side of the head, and 60 degrees from the center toward the nose). As is known from clinical experience with patients who are going blind from retinitis pigmentosa, a patient's ability to navigate within unfamiliar environments becomes limited when the full extent of the field of peripheral vision becomes less than ten degrees of visual angle. As such, a retinal prosthetic must provide at least ten degrees of peripheral vision, and preferably much more.
There is, however, a severe barrier to achieving larger degrees of peripheral vision. Fundamentally, the ability to achieve degrees of visual field depends upon the ability to stimulate the nerve cells within an area of retina that corresponds to that area of visual field. Ten degrees of visual field corresponds to roughly 3 mm over the retinal surface. Hence an array would have to cover at least a 3×3 mm region of retina to meet the most minimal criteria for delivering an adequate degree of benefit to severely blind patients. It would be preferable to provide substantially more peripheral vision than 10 degrees, given that 150 degrees is the normal amount of vision that humans can experience. Inserting large stimulating arrays is problematic, since an incision in the wall of the eye must be at least as large as the width of the stimulating array. Larger incisions are known to be associated with greater risk for surgical complications, and hence there would seem to be a practical limit on the degree of peripheral vision that could be achieved.
Solutions to the challenges in providing a relatively large degree of peripheral vision have been proposed in the prior art. Polyimide films with embedded microelectrodes have been used for neural recording in animal experiments (25). The present inventors and their associates first taught the use of such thin and flexible microfilms for use in human retinal stimulation (See U.S. Pat. No.: 6,324,429 to Shire et al.; U.S. Pat. No. 6,120,538 to Rizzo et al. and U.S. Pat. No. 5,800,530 to Rizzo, the teachings of which are incorporated herein by reference). These microfilms can be placed into an eye through a relatively small incision in the wall of the eye. Once inside of the eye, these films can be flattened onto the retina to provide a relatively large surface area of nerve stimulation.
Another solution, also taught by the present inventors, is the use of an inflatable microelectronic device (see U.S. Pat. No. 6,368,349 to Wyatt et al., the teachings of which are incorporated herein by reference), which can also be inserted through a small incision and then expanded within the eye so that the array contacts the epi- or sub-retinal surface. Further solutions are still needed to address the issues inherent in providing large degree peripheral vision.
Prior art retinal prostheses have been created through the various choices of stimulation location (i.e. epi- vs. sub-retinal), various means of powering the device (i.e. use of light vs. radiofrequency transmission), and various locations for obtaining an image (i.e., extraocularly or intraocularly). In particular, Chow et al. have taught the use of sub- or epi-retinal devices that use light for transmission to the prosthesis of spatially specific visual detail and power. (See, for example, U.S. Pat. Nos.: 6,075,251; 6,069,365; 6,020,593; 5,949,064; 5,895,415; 5,837,995; 5,556,423; and 5,397,350). Michelson, referenced above, has taught the use of an epi-retinal device that utilizes both light and radiofrequency transmission. Subsequently, Humayun et al. taught the use of epi-retinal devices that use radiofrequency transmission alone. (See U.S. Pat. No. 5,935,155). The teachings of all of the references cited above are incorporated herein by reference.
There is therefore still a need for an improved ocular device capable of more safely and effectively performing the needed functions of a retinal prosthesis by minimizing the anatomical disruption of the delicate interior of the eye while maximizing the safe delivery of the energy needed to drive a large number of stimulating elements.